CMOS image sensors can be manufactured at relatively low manufacturing costs as compared to Charge Couple Devices (CCDs), and thus are frequently used as the preferred type of solid-state image device. Because a unit pixel of a CMOS image sensor is formed with MOS transistors, a CMOS image sensor typically can be embodied within a smaller footprint area than that of a CCD. Furthermore, signal processing logic (e.g., circuits such as Correlated Double Sampling (CDS), Analog Digital Converter (ADC), and a ramp generator), can be formed simultaneously with the MOS transistors that constitute a unit pixel. Accordingly, the CMOS image sensor supports system-on-a-chip manufacturing.
A unit pixel of a CMOS image sensor includes a photodiode for sensing images, and transistors for transferring the signal sensed by the photodiode to a signal processing circuit. Such a unit pixel of the CMOS image sensor is shown in FIG. 1. Referring to FIG. 1, a transfer gate 15 is formed on a semiconductor substrate 10 where an active region is defined by an isolating layer 11. A photodiode 20 is formed in the active region on one side of the transfer gate 15, and a floating diffusion region 25 is formed in the active region on another side of the transfer gate 15. When the transfer gate 15 is formed, a gate of a reset transistor for resetting the floating diffusion region 25, a gate of a source follower transistor, and a gate of a select transistor may be simultaneously formed. The transfer gate 15 and the floating diffusion region 25 are respectively connected to a metal interconnect 30 and an external power source. The metal interconnect 30 includes multiple-layered metal interconnects 30a, 30b, and 30c. The metal interconnects 30a, 30b, and 30c are insulated from one another by respective interlayer insulating layers 35. The metal interconnect 30 are formed of a light blocking material and are disposed to surround a periphery of the photodiode 20. By allowing the metal interconnect 30 to surround the periphery of each photodiode 20, the pixels can be partitioned and somewhat shielded from each other.
However, as the resolution of the CMOS image sensor is increased, optical crosstalk may occur even though the metal interconnect 30 surrounds each photodiode 20 and acts as an optical shield.
More specifically, as shown in FIG. 1, the crosstalk is caused by obliquely incident light 50 that is incoming to an adjacent photodiode 20 through an interval among the metal interconnects 30a, 30b, and 30c disposed on the same plane, or due to mutual reflection among the upper and lower metal interconnects 30a, 30b, and 30c. Furthermore, when viewed from the same plane, the metal interconnects 30a, 30b, and 30c are spaced apart from the photodiode 20 by a predetermined distance. Therefore, the light is scattered and the quantity of the obliquely incident light is increased, so that the degree of crosstalk can be severe. Such crosstalk may mix data to cause color blurring, and brighten surrounding pixels when a bright image is scanned. In FIG. 1, a reference character {circle around (e)} denotes charge generated by the photodiode 20, in response to receipt of optical information. Accordingly, obliquely incident light cannot be thoroughly blocked by the metal interconnect 30 surrounding the photodiode 20.